U.S. Pat. No. 4,116,704 disclosed the production of transparent glass articles exhibiting a critically-defined, light gray-brown tint having compositions within a specifically-delineated area within the sodium aluminoborosilicate system containing narrowly-restricted amounts of Co.sub.3 O.sub.4, NiO, and MnO.sub.2 to impart the desired gray-brown tint thereto. Thus, the patented glasses followed a particular transmittance curve in the visible portion of the radiation spectrum and consisted essentially, expressed in terms of parts by weight on the oxide basis, of about:
SiO.sub.2 --78.0-80.0 PA1 B.sub.2 O.sub.3 --14.16-12.35 PA1 Na.sub.2 O--5.15-4.7 PA1 Al.sub.2 O.sub.3 --2.21-2.0 PA1 Co.sub.3 O.sub.4 --0.001-0.006 PA1 NiO--0.037-0.055 PA1 MnO.sub.2 --0.21-0.6
Customarily, those glasses will be chloride fined via the addition of up to about 1% NaCl.
The mechanism by which the Ni.sup.+2 ion imparts color to a glass depends upon whether it is present in a four-fold or six-fold coordination, as is explained in U.S. Pat. No. 4,116,704, and that discussion is explicitly incorporated herein by reference. In brief, when in the four-fold coordination, Ni.sup.+2 ions confer a purple coloration to the glass, and, when in the six-fold coordination, Ni.sup.+2 ions provide a yellow tint to the glass. Thermal treatment of the glass influences the coordination state adopted by the Ni.sup.+2 ions; e.g., conventional annealing of the glass results in the Ni.sup.+2 ions being in the six-fold coordination, thereby promoting a yellow coloration in the glass. In contrast, quick cooling of the glass, as occurs in thermal tempering, leaves the Ni.sup.+2 ion in the four-fold coordination. And because borosilicate glasses utilized in culinary ware are customarily thermally tempered to enhance mechanical strength and thermal shock resistance, this capability of Ni.sup.+2 ions to assume six-fold or tetrahedral coordination is of vital practical significance to the glass manufacturer.
Because the cost of boron-containing glass batch ingredients and, particularly, boric acid has escalated rapidly in recent years, there has been the desire to reduce the overall amount of B.sub.2 O.sub.3 in the glass and/or modify the glass composition such that a less expensive raw material, e.g., borax, could be substituted for boric acid. The base glass compositions of U.S. Pat. No. 4,116,704 were modified so as to reduce the B.sub.2 O.sub.3 content while retaining comparable chemical and physical properties. However, in doing so, two important differences were soon observed existing between the glasses of U.S. Pat. No. 4,116,704 and those newly-devised.
First, the changes in composition altered the "molecular" structure of the glass. Thus, the ratio on a molar basis (moles of substance) of Na.sub.2 O:B.sub.2 O.sub.3 has been regarded as an important parameter in the chemical and physical behavior of borosilicate glasses. See "Borate Glasses--Structure, Preparation, and applications", L. D. Pye, V. D. Frechette, and N. J. Kreidl, Plenum Press, New York, 1978. The molar ratio Na.sub.2 O:B.sub.2 O.sub.3 of the compositions of U.S. Pat. No. 4,116,704 are generally less than 0.47. In contrast, the molar ratio Na.sub.2 O:B.sub.2 O.sub.3 of the inventive glasses will customarily be at least 0.5. And, as is explained in that patent, structural changes occurring in glass strongly influence the colors obtained from the inclusion therein of various transition metal ions, especially Ni.sup.+2 ions.
The second major difference existing between the glasses of U.S. Pat. No. 4,116,704 and those recently prepared lies in the level of NaCl required to effect fining of the glass. Hence, the patent prescribes the incorporation of up to about 1% NaCl. In contrast, the newly-devised glasses employ no more than about 0.5% NaCl with typical contents ranging between about 0.2-0.4%. This reduction in NaCl level in the batch also results in changes in the colors displayed by various transition metal oxide colorants.
Consequently, for those two reasons the combination of NiO, MnO.sub.2 and Co.sub.3 O.sub.4 utilized in U.S. Pat. No. 4,116,704 to secure the desired light gray-brown hue in the recited base Na.sub.2 O-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -SiO.sub.2 glasses imparts colors which are too dark, i.e., having a lower Y.sub.c value using the C.I.E. notation, and too purple, thereby indicating a higher proportion of tetrahedrally coordinated Ni.sup.+2 ion being present. Stated in a different manner, it is not possible to match the chromaticity and luminous transmittance targets of the glasses of U.S. Pat. No. 4,116,704 utilizing any combination of NiO, MnO.sub.2, and Co.sub.3 O.sub.4 with the lower B.sub.2 O.sub.3 base glasses.